The mixture of fecal matter and water is commonly referred to as “sewage” or “wastewater” by sanitary engineers. For hundreds of years, mankind used flowing water to transport sewage elsewhere so as to be able to live in more pleasant surroundings. Many cities built sewers to convey sewage into bodies of water. These bodies of water could be freely flowing, such as rivers, and streams; or terminal, such as seas, lakes, and ponds. For example, sanitary waste was disposed of in Boston's early storm-sewer system, which discharged the untreated waste into the Charles River (then Boston's water supply) and Boston Harbor. Additionally, sewers were built that conveyed sewage directly onto dry land.
Sewers today go beyond just dumping sewage into a waterway. They now treat the sewage before it is put back into the ecosystem. Expansive sewage treatment facilities separate the solids and treat the sewage with bacteria and chemicals that make it safe to introduce back into the environment. Solids left over from the treatment process are even reused as fertilizer.
Fecal material biodegrades via aerobic and anaerobic processes, depending on whether oxygen is present. Sewage treatment facilities use, among other things, bacteria to digest sewage. Bacteria ingest the organic material in sewage to grow and produce more bacteria. Along with the bacteria, carbon dioxide and water are the principal end-products of aerobic sewage degradation. Following the degradation process, the oxygen content of the water will be decreased. The rate of bacterial growth is limited by the amount of oxygen available. For decades, sanitary engineers have sought to increase or “speed up” aerobic degradation processes by increasing the amount of oxygen available. One such method is referred to as “activated sludge treatment,” or “AST”.
Activated Sludge Treatment
The activated sludge treatment process is employed in more than 80% of all treatment plants in the field of home, municipal, and biodegradable industry wastewater treatment.
The basic principle behind all activated sludge processes is that as microorganisms grow, they form particles that clump together. These particles (“floc”) are allowed to settle to the bottom of a tank, leaving a relatively clear liquid free of organic material and suspended solids. Described simply, wastewater is mixed with varying amounts of recycled liquid containing a high proportion of organisms taken from a secondary clarifying tank, and it becomes a product called mixed liquor. This mixture is typically stirred and injected with large quantities of air, to provide oxygen and keep solids in suspension. After a period of time, mixed liquor flows to a clarifier where it is allowed to settle. A portion of the bacteria is removed as it settles, and the partially cleaned liquid flows on for further treatment. The resulting settled solids, the so-called “activated sludge,” are returned to the first tank to begin the process again.
Broadly speaking, the basic activated sludge process typically takes place in a system comprising several interrelated components, including, for example: (1) a tank where the biological reactions occur, sometimes referred to as an aeration tank; (2) a source that provides oxygen and mixing; (3) a tank, known as the clarifier, where the solids settle and are separated from treated wastewater; and (4) a means of collecting the solids either to return them to the aeration tank (in the form of “return activated sludge” or “RAS”), or to remove them from the process (known as “waste activated sludge” or “WAS”).
AST is frequently employed in a multi-part wastewater treatment (“WWT”) system. Such systems include plug flow reactors, continuously stirred tank reactors, and mixed-flow reactors. One or more reactors can be used. While it is most common to use only one type of reactor in a particular plant, different types of reactors can be used in the same plant. In the case where there is more than one reactor in a WWT plant, the reactors may be connected in series, in parallel, or in a series/parallel connection. The steps that take place in this system are usually referred to as “primary treatment,” “secondary treatment,” and “tertiary treatment.” These steps are typically used in series, from first to last; but all of the steps may not be used in all cases, and steps may be repeated as desired.
Additional steps may be also included, e.g., pre-treatment to remove materials that can be collected from the raw sewage before they damage or clog the pumps and sewage lines of primary treatment clarifiers. Objects commonly removed during pre-treatment include trash, tree limbs, leaves, branches, and other large objects. In New York City, for example, several stories underground, wastewater flows into the plants from sewers connected to homes and businesses. The incoming wastewater, called influent, passes through screens consisting of upright bars, spaced one to three inches apart. These bars remove large pieces of trash including rags, sticks, newspaper, soft drink cans, bottles, plastic cups and other similar items. This protects the main sewage pumps and other equipment. The garbage is transported to landfills. The main sewage pumps then lift the wastewater from the screening chamber to the surface level of the plant. See “New York City's Wastewater Treatment System,” available at: http://www.nyc.gov/html/dep/html/wastewater/wwsystem-process.shtml.
As will be well-known to one of ordinary skill in the art, the term “activated sludge treatment” can be used to describe several steps in the process (e.g., both the primary and secondary treatments together, or the biological growth portion of secondary treatment stage alone).
A number of variations of the basic process have been developed, including extended aeration, sequencing batch reactors, and oxidation ditches. The activated sludge plant is the most popular biological treatment process for larger installations or small package plants being used today. These plants are capable of producing a high quality effluent for the price. Another advantage of the activated sludge process is the low construction cost.
The activated sludge treatment process has several disadvantages, though. The efficiency of treatment varies widely during the course of any treatment process, giving widely-varying pollution results. Additionally, depending on the aeration method employed, the process can be very expensive.
Therefore, an activated sludge treatment system and process that overcome these and other drawbacks in the art is desired. Additionally, an activated sludge treatment system and process that requires less energy input and as a result, costs less, is also desired.
Primary Treatment. In primary treatment, raw sewage is mechanically sorted to remove the solid part of the sewage stream from the liquid part. Primary treatment is also sometimes referred to by those skilled in the art as the primary sedimentation stage.
In one example, sewage flows through large tanks, commonly called pre-settling basins, primary chambers, primary sedimentation tanks, or primary clarifiers. Primary settling tanks can be equipped with mechanically-driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities, where, for example, it can be dried and sterilized and disposed of, usually by land-filling. Removing suspended solid waste reduces the wastewater's biochemical oxygen demand (“BOD”)—the amount of oxygen microorganisms must consume to breakdown the organic material present in the wastewater.
In New York City, for example, the wastewater (following pre-treatment) enters primary settling tanks or sedimentation tanks, for one to two hours. The flow of the wastewater is slowed, allowing heavier solids to settle to the bottom of the tank and the lighter materials to float. At the end of the process, the floatable trash, such as grease and small plastic material, rises and is skimmed from the top of the tank's surface. The settled solids, called primary sludge, are then pumped through cyclone degritters—devices that use centrifugal force to separate out sand, grit (such as coffee grinds) and gravel. This grit is removed, washed, and taken to landfills. The degritted primary sludge is pumped to the plant's sludge handling facilities for further processing. The partially treated wastewater from the primary setting tanks (called primary-treated wastewater) then flows to the secondary treatment system.
Secondary Treatment. In secondary treatment, wastewater undergoes a biological process to remove dissolved and suspended organic compounds. After this kind of treatment, the wastewater may be called secondary-treated wastewater.
The process typically includes a liquid sewage stream that flows to a managed aerobic habitat wherein the stream's organic compounds are sequestered by being ingested by naturally-occurring microbes like bacteria and protozoa, which convert biodegradable soluble organic contaminants (e.g., sugars, fats, and organic short-chain carbon molecules from human waste, food waste, soaps and detergent) into, e.g., carbon dioxide and water. These microbes clump together and form floating particles. This biological process takes place in a suitable reactor that includes a means of adding oxygen to facilitate aerobic bacterial growth, and a means of keeping the sludge particles in suspension (e.g., a mixing basin or chamber, or aeration tank). Overflow from the mixing basin may be sent to a secondary clarifier, where the suspended biological floc settles out while the treated water moves into tertiary treatment or disinfection. The settled sludge can be sent to terminal processing and disposal. Settled sludge can also be returned to the mixing basin to continue growing in primary effluent. The returned sludge is called “return activated sludge” or “RAS.”
In New York City, for example, in the secondary treatment process, air and “seed” sludge are added to the wastewater to break it down further. Air pumped into large aeration tanks mixes the wastewater and sludge that stimulates the growth of oxygen-using bacteria and other organisms that are naturally-present in the sewage. These beneficial microorganisms consume most of the remaining organic materials that are polluting the water and this produces heavier particles that will settle later in the treatment process. Wastewater then passes through these bubbling tanks in three to six hours. The aerated wastewater then flows to the final settling or sedimentation tanks, which are similar to the primary settling tanks. Here, the heavy particles and other solids settle to the bottom as secondary sludge. Some of this sludge is re-circulated back to the aeration tanks as “seed” to stimulate the activated sludge process. The returned sludge contains millions of microorganisms that help maintain the right mix of bacteria and air in the aeration tank and contribute to the removal of as many pollutants as possible. The remaining secondary sludge is removed from the settling tanks and added to the primary sludge for further processing in the sludge handling facilities. Wastewater passes through the settling tanks in two to three hours and then flows to the tertiary treatment stage (e.g., filtration or disinfection.)
The secondary treatment process may encompass a variety of mechanisms and processes using dissolved oxygen to promote growth of the biological floc that removes organic material. But, as discussed above, biological oxidation processes are sensitive. For example, the rate of biological reactions increase with temperatures between 0° C. and 40° C. Most surface aerated vessels operate at between 4° C. and 32° C. Elevated concentrations of toxic wastes including pesticides, industrial metal plating waste, or extreme pH, can kill the biota of an activated sludge reactor ecosystem.
The amount of oxygen available during the secondary treatment process is also important. Most secondary treatment processes include an aeration step. Aeration serves two important purposes: supplying the required oxygen to the organisms to grow and providing optimum contact between the dissolved and suspended organic matter and the microorganisms, by driving the cross roll current that keeps the floc suspended. The aeration system consumes approximately 70 to 80 percent of the net power demand for a typical activated sludge wastewater treatment plant; therefore, the efficiency of different aeration systems is an important consideration. The time that the mixed liquor is aerated varies from as little as 30 minutes to as much as 36 hours, depending upon the treatment process used. The quality of wastewater that is released after treatment is measured in part by the amount of suspended solids or “SS” present in solution, usually in milligrams per liter. The EPA has standards for BOD and SS discharge from wastewater treatment plants.
Aeration is usually performed mechanically or by using a diffused system. Mechanical aerators physically splash the wastewater into the atmosphere above the tank and create turbulence causing wastewater mixing. Mechanical aerators include brushes, blades, or propellers that introduce air from the atmosphere. Surface aerators float at the surface or are mounted on supports in or above the basin. Mechanical aerators tend to incur lower installation and maintenance costs. A diffused air system introduces compressed air through a perforated membrane into the wastewater. Diffusers are classified by the physical characteristics of the equipment, or by the size of the air bubble. The choice of bubble size, diffuser type, and diffuser placement can have a great effect on the efficiency of the aeration process. Porous (fine bubble) diffusers are attached to the bottom of the tank or positioned just below the surface. They are available in various shapes and sizes, such as discs, tubes, domes, and plates. Fine pore diffusers introduce air in the form of very small bubbles, maximizing the contact time the air bubbles have with the mixed liquor and encouraging mixing, while at the same time, discouraging deposits on the tank bottom. These fine pore diffusers produce a high oxygen transfer efficiency, but they are susceptible to chemical or biological fouling and as a result, require routine cleaning. Nonporous (course bubble) diffusers usually have fixed or valved orifices. Due to the larger bubble size, nonporous diffusers produce lower oxygen transfer efficiencies. Other diffusion devices include aspirator aerators that use a propeller on the end of a hollow shaft, creating a vacuum as the propeller draws air from the atmosphere and disperses it into the wastewater.
Tertiary Treatment. The highest level of wastewater treatment is tertiary treatment, which is any process that goes beyond the previous steps to further remove contaminants or specific pollutants. Even after primary and secondary treatment, disease-causing organisms may remain in the treated wastewater. In tertiary treatment, the secondary effluent can be polished by physical processes, biological processes, or a combination thereof. For example, the most common tertiary treatment is filtration. Another example is disinfection. Tertiary treatment is typically used to remove phosphorous or nitrogen, which cause eutrophication. In some cases, treatment plant operators add chlorine as a disinfectant before discharging the water. Tertiary treatment can produce potable water.
In New York City, for example, to disinfect and kill harmful organisms, the wastewater spends a minimum of 15-20 minutes in chlorine-contact tanks mixing with sodium hypochlorite. The treated wastewater, or effluent, is then released into local waterways.